Open your mouth and take a deep breath, then breathe back out onto your hand. Does your hand feel moist? Plants have more stomata only if they can afford to lose that much moisture , fewer stomata if they must conserve water . Are you new to botany? In case you’re a bit green, let’s take a step back.
What are stomata, exactly?
Stomata are like the mouths of plants, except that they can have many hundreds of “mouths” per leaf where we only have one for our whole body. Recall that:
- Plants use carbon dioxide (CO2) that they “breathe in” to make sugars (plant food).
- A byproduct of that process is oxygen , which the plants ” breathe out”.
- This “breathing” is actually passive flowing of gasses in and out of the stomata.
- There is water in the air, but more water in the plant’s leaves.
- Every time a stoma opens, water vapor is one of the gasses that leaves… the leaves.
So we now know that stomata are critical to the plant as they get essential molecules from the air into the leaf parts that make plant food. Stomata open their lips (guard cells) when the plant’s chemical process es signal that more CO2 is needed. You’ll recognize the name of the mechanism by which plants make sugar: Photosynthesis. As you likely know, photosynthesis requires sunlight. Adding that piece to the puzzle, we start to see why desert plants must really be special. If plants make food when it’s light out, in the heat of the day, that is when they’ll be opening their stomata for more CO2 – gulping in air and spitting out water. But “the heat of the day” in the desert is so hot and dry!
How do plants survive in the desert?
Although plants get plenty of sun light in the desert for photosynthesis, it’s actually too much. Extra heat and low humidity mean that water will pass out of the plant that much faster. Also, there isn’t as much water in the soil, so the plant cannot reliably replace water that is lost. Because of this, you’ll find that anyone who’ s anyone in the community of desert plants will share many of the following traits:
- Waxy coating to help keep water in the plant.
- Thick, small leaves that reduce the amount of surface area exposed to heat.
- Large pockets for absorbing water that can then be saved for a rainy… eh, a not-so-rainy day.
- Thorns to keep animals from eating the plant to drink that stored water.
- Limiting certain critical phases (growth, reproduction) to short periods of the year.
But my favorite desert plant adaptation by far is that of hording. Many desert plants use an alternative photosynthesis mechanism called “CAM” photosynthesis (Crassulacean Acid Metabolism). Photosynthesis can only occur when tons of CO2 flood the inside of the plants’ cells. Rather than leave the stomata gaping during the day to provide that CO2 , CAM plants suck in CO2 at night and stockpile it until day. At that time, it is released in saturating bursts to the right plant parts when the sun is up, without the stomata having to open in the heat.
So, do desert plants have fewer stomata?
Yes. Although many of these other adaptations, including CAM photosynthesis, make the stomata less burdensome, desert plants still have fewer per square inch. One addition, more subtle desert plant adaptation is that many desert plants grow much slower and are just… well… lazy. To increase food production by increasing stomata, at the risk of losing more water than can be regained, is not energetically worth it. The plant would not live to reproduce and, thus, nature has made its selection.